The geocentric theory is the belief that the Earth is the center of the universe and that all other

objects orbit around it. This outdated and disproved theory is often attributed to Ptolemy. The

belief was very popular in China and ancient Greece and widely expounded on by the leading

thinkers of the day: Aristotle and Ptolemy. The theory was thought to be proved by to

observations: that the stars, sun, and planets appear to revolve around the Earth each day, with

the stars circling around the pole and those stars nearer the equator rising and setting each day

and circling back to their rising point. Second, since the Earth is solid and stable, it is not moving,

so being at rest other bodies must orbit around it. Geocentrism was not replaced until the 1600′s.

Clausius Ptolemy is the man widely credited with taking the geocentric theory mainstream. A few

of the basic tenets had been published since the 6th century B.C and all of the basic components

were established by Aristole’s time. Ptolemy’s influence was so extensive that his name became

synonymous with geocentrism. Despite the popularity of the model, not everyone agreed with it.

The Pythagorean school believed that the Earth was one of several planets going around a

central fire. Martianus Capella put Mercury and Venus on epicycles around the Sun. Aristarchus

of Samos wrote a work on heliocentrism saying that the Sun was at the center of the universe

and the Earth and other planets revolved around it. His theory was not popular and he had one

known follower. It was not until 1563, when Copernicus published his works, that a more

workable heliocentric view was postulated. Still, it took until 1610, when Galileo Galilei was able

to offer a more verifiable theory by using a telescope, for the heliocentric view to begin to

overtake geocentrism.

What truly disproved the geocentric theory was the invention of the telescope. Observations with

the unaided eye were too limited to show the true motions in the heavens. With the knowledge

that we have accumulated; it is hard to understand that anyone ever believed in geocentrism, but

we owe much of our knowledge base to the early theorists.

We have written many articles about the Geocentric Theory for Universe Today. Here’s an article

about the Geocentric Model, and here’s an article about the difference between the Geocentric

and Heliocentric Model.

If you’d like more info on the Geocentric Theory, check out an article about the Geocentric

Theory, and here’s a link to an article about the center of the galaxy.

We’ve also recorded a series of episodes of Astronomy Cast about the center of the universe.

Listen here, Episode 77: Where is the Centre of the Universe.

Tagged as: clausius ptolemy, geocentric theory, geocentrism, ptolemy

Heraclides (330 B.C.) developed the first Solar System model, beginning of the geocentric versus heliocentric debate

Note that orbits are perfect circles (for philosophical reasons = all things in the Heavens are "perfect")

Aristarchus (270 B.C.) developed the heliocentric theory

Problems for heliocentric theory:

Earth in motion??? can't feel it

no parallax seen in stars geocentric = ego-centric = more "natural" The early Greeks observed the sky and all that it contained. From their

observations, the Greeks believed the Earth was the center of the moon, Sun, and the only known planets at that time, Mercury, Venus, Mars, and Jupiter. These planets were said to be moving around Earth in a clockwise direction. They believed the Earth was motionless, because no one felt the Earth moving. The stars appeared to move around the Earth daily, further convincing them of this theory, which became known as geocentric or Earth-centered. The Greeks had a basic understanding of geometry and trigonometry, which lead them to conclude that fast moving objects were closer to the Earth than slower moving objects.

Around 140 A.D., Claudius Ptolemy wrote thirteen volumes on the motion of the planets, and put the geocentric theory in its finest form. The models in Ptolemy's volumes became known as the Ptolemaic system. In these volumes, Ptolemy discusses epicycles, deferent, and retrograde. Epicycles are tiny circles the planets orbit on. The center of an epicycle would move along a large circle, known as the deferent. Ptolemy used epicycles in his models to make retrograde more visible. The combination of counter-clockwise motion, epicycles, and deferent create the motion of retrograde. Ptolemy explained retrograde motion, as an apparent westward drift. This westward drift of the planets would give one the impression that it was backing up.

The Roman Catholic church accepted the Ptolemaic theory, because biblical passages suggested the sun was in constant motion while Earth remained in one place. Since the Church was in control during this time period, anyone who did not believe in the Ptolemaic theory would be punished, possibly with house arrest.

Introduction: On the Philosophy & Metaphysics of Charles Darwin's Theory of Evolution

For thousands of years many philosophers had argued that life must have been created by a supernatural being / creator / God due to the incredible complexity of Nature (in particular, we humans and our minds). Thus it is remarkable that Charles Darwin (and others) were able to explain our existence by means of Evolution from Natural Selection - which is veryobvious once understood.

Below you will find a brief summary of Charles Darwin's Theory of Evolution and some interesting quotes from Darwin on Evolution, Natural Selection, Science, Humanity, God and Religion.

When thinking about evolution, it is important to take a further step and ask, 'What is evolving?' As this website explains, there is a simple and obvious explanation of what exists and thus how we can understand the metaphysical foundations of Evolution. See Evolution-Metaphysicswebpage.

We hope you enjoy the following quotes and browsing around this website. We have a wonderful collection of knowledge from many of the greatest minds of human history - and most importantly can provide a simple sensible explanation for most of them!Cheers,Geoff Haselhurst, Karene Howie

Although I am fully convinced of the truth of the views given in this volume I by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. But I look with confidence to the future to young and rising naturalists, who will be able to view both sides of the question with impartiality. (Charles Darwin)

Introduction - Charles Darwin's Theory of Evolution - Darwin Quotes on Evolution - Evolution Articles - Links / Evolution - Top of Page

 Charles Darwin's Theory of EvolutionBrief Summary

Darwin's theory of evolution is based on five key observations and inferences drawn from them. These observations and inferences have been summarized by the great biologist Ernst Mayr as follows:

1) Species have great fertility. They make more offspring than can grow to adulthood.

2) Populations remain roughly the same size, with modest fluctuations.

3) Food resources are limited, but are relatively constant most of the time.

From these three observations it may be inferred that in such an environment there will be a struggle for survival among individuals.

4) In sexually reproducing species, generally no two individuals are identical. Variation is rampant.

5) Much of this variation is heritable.

From this it may be inferred: In a world of stable populations where each individual must struggle to survive, those with the "best" characteristics will be more likely to survive, and those desirable traits will be passed to their offspring. These advantageous characteristics are inherited by following generations, becoming dominant among the population through time. This is natural selection. It may be further inferred that natural selection, if carried far enough, makes changes in a population, eventually leading to new species. These observations have been amply demonstrated in biology, and even fossils demonstrate the veracity of these observations.

To summarise Darwin's Theory of Evolution;1. Variation: There is Variation in Every Population.2. Competition: Organisms Compete for limited resources.3. Offspring: Organisms produce more Offspring than can survive.4. Genetics: Organisms pass Genetic traits on to their offspring.5. Natural Selection: Those organisms with the Most Beneficial Traitsare more likely to Survive and Reproduce.

Darwin imagined it might be possible that all life is descended from an original species from ancient times. DNA evidence supports this idea. Probably all organic beings which have ever lived on this earth have descended from some one primordial life form. There is grandeur in this view of life that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved. (Charles Darwin, The Origin of Species)

Edited from http://en.wikipedia.org/wiki/Charles_Darwin

Introduction - Charles Darwin's Theory of Evolution - Darwin Quotes on Evolution - Evolution Articles - Links / Evolution - Top of Page

 Charles Darwin QuotesTheory of Evolution, Science, Humanity, Knowledge, God &

Religion

In scientific investigations, it is permitted to invent any hypothesis and, if it explains various large and independent classes of facts, it rises to the rank of a well-grounded theory. (Charles Darwin)

How extremely stupid for me not to have thought of that!(Thomas Huxley's first reflection after mastering, in 1859, the central idea ofDarwin's Origin of Species)

Ignorance more frequently begets confidence than does knowledge: it is those who know little, not those who know much, who so positively assert that this or that problem will never be solved by science. (Charles Darwin, Introduction to The Descent of Man, 1871)

In the struggle for survival, the fittest win out at the expense of their rivals because they succeed in adapting themselves best to their environment. (Charles Darwin)

Man with all his noble qualities, with sympathy which feels for the most debased, with benevolence which extends not only to other men but to the humblest living creature, with his god-like intellect which has penetrated into the movements and constitution of the solar system- with all these exalted powers- Man still bears in his bodily frame the indelible stamp of his lowly origin. (Charles Darwin)

Nothing before had ever made me thoroughly realise, though I had read various scientific books, that science consists in grouping facts so that general laws or conclusions may be drawn from them. (Charles Darwin)

I have no great quickness of apprehension or wit which is so remarkable in some clever men, for instance Huxley. (Charles Darwin)

We will now discuss in a little more detail the Struggle for Existence... The expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient. (Charles Darwin)

.. doing what little one can to increase the general stock of knowledge is as respectable an object of life, as one can in any likelihood pursue. (Charles Darwin)

a scientific man ought to have no wishes, no affections .. a mere heart of stone. (Charles Darwin)

I am turned into a sort of machine for observing facts and grinding out conclusions. (Charles Darwin)

The fact of evolution is the backbone of biology, and biology is thus in the peculiar position of being a science founded on an improved theory, is it then a science or faith? (Charles Darwin)

Charles Darwin on God / Religion

I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of their feeding within the living bodies of Caterpillars. (Charles Darwin)

As for a future life, every man must judge for himself between conflicting vague probabilities. (Charles Darwin)

Believing as I do that man in the distant future will be a far more perfect creature than he now is, it is an intolerable thought that he and all other sentient beings are doomed to complete annihilation after such long-continued slow progress. (Charles Darwin)

We can allow satellites, planets, suns, universe, nay whole systems of universes to be governed by laws, but the smallest insect, we wish to be created at once by special act. (Charles Darwin)

I am a strong advocate for free thought on all subjects, yet it appears to me (whether rightly or wrongly) that direct arguments against christianity and theism produce hardly any effect on the public; and freedom of thought is best promoted by the gradual illumination of men's minds, which follow[s] from the advance of science. It has, therefore, been always my object to avoid writing on religion, and I have confined myself to science. I may, however, have been unduly biased by the pain which it would give some members of my family, if I aided in any way direct attacks on religion. (Charles Darwin)

http://www.darwin-literature.com/l_quotes.html

When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Cambrian system was deposited, they seem to me to become ennobled. (Charles Darwin, The Origin of Species, quoted from John Stear, No Answers in Genesis)

What a book a Devil's Chaplain might write on the clumsy, wasteful, blundering low and horribly cruel works of nature. (Charles Darwin, quoted by Richard Dawkins in A Devil's Chaplain, 2004)

When it was first said that the sun stood still and world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei [the voice of the people is the voice of God], as every philosopher knows, cannot be trusted in science.(Charles Darwin, reminding his readers that they should always treat "obvious" truths with skepticism, in the context of the apparent absurdity of evolving a complex eye through a long series of gradual steps, in the famous passage added to later editions of the Origin of Species (1872, p. 134), quoted from Stephen Jay Gould, The Structure of Evolutionary

Theory (2002), chapter 1, "Defining and Revising the Structure of Evolutionary Theory," p. 1 (the bracketed translation is Gould's)

False facts are highly injurious to the progress of science, for they often endure long; but false views, if supported by some evidence, do little harm, for everyone takes a salutary pleasure in proving their falseness; and when this is done, one path toward errors is closed and the road to truth is often at the same time opened. (Charles Darwin, The Descent of Man)

A celebrated author and divine has written to me that he has gradually learned to see that it is just as noble a conception of the Deity to believe that he created a few original forms capable of self-development into other and needful forms, as to believe that he required a fresh act of creation to supply the voids caused by the action of his laws. (Charles Darwin, Origin of Species p. 422)

About thirty years ago there was much talk that geologists ought only to observe and not theorize; and I well remember someone saying that at this rate a man might as well go into a gravel-pit and count the pebbles and describe the colours. How odd it is that anyone should not see that all observation must be for or against some view if it is to be of any service!(Charles Darwin, letter to Henry Fawcett, who had defended Darwin before the British Association for the Advancement of Science against a critic who said Darwin's book was too theoretical and that he should have just "'put his facts before us and let them rest," quoted from Michael Shermer, "Colorful Pebbles and Darwin's Dictum: Science is an exquisite blend of data and theory," Scientific American, May, 2001)

How so many absurd rules of conduct, as well as so many absurd religious beliefs, have originated, we do not know; nor how it is that they have become, in all quarters of the world, so deeply impressed on the minds of men; but it is worthy of remark that a belief constantly inculcated during the early years of life, while the brain is impressionable, appears to acquire almost the nature of an instinct; and the very essence of an instinct is that it is followed independently of reason. (Charles Darwin, Descent of Man p. 122)

I am aware that the assumed instinctive belief in God has been used by many persons as an argument for his existence. The idea of a universal and beneficent Creator does not seem to arise in the mind of man, until he has been elevated by long-continued culture. (Charles Darwin, Descent of Man p. 612)

I am aware that the conclusions arrived at in this work will be denounced by some as highly irreligious; but he who denounces them is bound to show why it is more irreligious to explain the origin of man as a distinct species by descent from some lower from, through the laws of variation and natural selection, than to explain the birth of the individual through

the laws of ordinary reproduction. The birth both of the species and of the individual are equally parts of that grand sequence of events, which our minds refuse to accept as the result of blind chance. (Charles Darwin, Descent of Man p. 613)

But I own that I cannot see as plainly as others do, and I should wish to do, evidence of design and beneficence on all sides of us. There seems to me too much misery in the world. I cannot persuade myself that a beneficent and omnipotent God would have designedly created that a cat should play with mice. (Charles Darwin, source unknown)

http://www.positiveatheism.org/hist/quotes/darwin.htm

Introduction - Charles Darwin's Theory of Evolution - Darwin Quotes on Evolution - Evolution Articles - Links / Evolution - Top of Page

 Evolution ArticlesQuotes from 'The Question of Questions' by Thomas H

Huxley, 'Evolution by Natural Selection and Buddhism' by Derek Freeman

The different branches of science combine to demonstrate that the universe in its entirety can be regarded as one gigantic process, a process of becoming, of attaining new levels of existence and organization, which can properly be called a genesis or an evolution. (Thomas Huxley)

.. no absolute structural line of demarcation .. can be drawn between the animal world and ourselves. (Thomas Huxley, 1863)

Abuse for six or seven years on the part of the public is of not the greatest consequence when one happens to be in the right and stands to one's guns. (Thomas Huxley)

Man has worked his way to the headship of the sentient world and has become the dominant animal that he is, by virtue of his success in the struggle for existence; and, in this struggle- as among other animals - it is self-assertion, the unscrupulous seizing upon all that can be grasped, the tenacious holding of all that can be kept, that have mattered. (Thomas Huxley)

In the Origin of Species, Darwin had not, in fact, discussed the bearing of Evolution theory on the human species, other than to remark that 'Light will be thrown on the origin of man and his history.' (Freeman)

Huxley was the first to construct, on the basis of Darwin's theory of evolution by natural selection, a clear and logical image of biological man, and as such, is clearly the founder of evolutionary anthropology. .. For Huxley, the notion that evolution can provide a foundation to morals was 'an illusion'. (Freeman)

This article is about the historical term. For modern geocentrism, see Modern geocentrism.

"Geocentric" redirects here. For orbits around the Earth, see Geocentric orbit.

Figure of the heavenly bodies — An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568 (Bibliothèque Nationale, Paris)

In astronomy, the geocentric model (also known as geocentrism, or the Ptolemaic system), is the superseded theory, that the Earth is the center of the universe, and that all other objects orbit around it. This geocentric model served as the predominant cosmological system in many ancient civilizations such as ancient Greece. As such, most Ancient Greek philosophers assumed that the Sun, Moon, stars, and naked eye planets circled the Earth, including the noteworthy systems of Aristotle (see Aristotelian physics) and Ptolemy.[1]

Two commonly made observations supported the idea that the Earth was the center of the Universe. The first observation was that the stars, sun, and planets appear to revolve around the Earth each day, making the Earth the center of that system. Further, every star was on a "stellar" or "celestial" sphere, of which the earth was the center, that rotated each day, using a line through the north and south pole as an axis. The stars closer the equator appeared to rise and fall the greatest distance, but each star circled back to its rising point each day.[2] The second common notion supporting the geocentric model was that the Earth does not seem to move from the perspective of an Earth bound observer, and that it is solid, stable, and unmoving. In other words, it is completely at rest.

The geocentric model was usually combined with a spherical Earth by ancient Greek and medieval philosophers. It is not the same as the older flat Earth model implied in some

mythology. However, the ancient Greeks believed that the motions of the planets were circular and not elliptical, a view that was not challenged in Western culture before the 17th century through the synthesis of theories by Copernicus and Kepler.

The astronomical predictions of Ptolemy's geocentric model were used to prepare astrological charts for over 1500 years. The geocentric model held sway into the early modern age, but was gradually replaced from the late 16th century onward by the heliocentric model of Copernicus, Galileo and Kepler. However, the transition between these two theories met much resistance, not only from the Catholic Church and its reluctance to accept a theory not placing God's creation at the center of the universe, but also from those who saw geocentrism as a fact that could not be subverted by a new, weakly justified theory.

Contents

[hide]

1 Ancient Greece 2 Ptolemaic Model

o 2.1 Ptolemaic system o 2.2 Geocentrism and Islamic astronomy

3 Geocentrism and rival systems o 3.1 Copernican system

4 Gravitation 5 Reluctance to change 6 Modern geocentrism

o 6.1 Planetariums 7 Geocentric models in science fiction 8 See also 9 Notes 10 References 11 External links

Ancient Greece

Illustration of Anaximander's models of the universe. On the left, daytime in summer; on the right, nighttime in winter.

The geocentric model entered Greek astronomy and philosophy at an early point; it can be found in Pre-Socratic philosophy. In the 6th century BC, Anaximander proposed a cosmology with the Earth shaped like a section of a pillar (a cylinder), held aloft at the center of everything. The Sun, Moon, and planets were holes in invisible wheels surrounding the Earth; through the holes, humans could see concealed fire. About the same time, the Pythagoreans thought that the Earth was a sphere (in accordance with observations of eclipses), but not at the center; they believed that it was in motion around an unseen fire. Later these views were combined, so most educated Greeks from the 4th century BC on thought that the Earth was a sphere at the center of the universe.[3]

In the 4th century BC, two influential Greek philosophers wrote works based on the geocentric model. These were Plato and his student Aristotle. According to Plato, the Earth was a sphere, stationary at the center of the universe. The stars and planets were carried around the Earth on spheres or circles, arranged in the order (outwards from the center): Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, fixed stars, with the fixed stars all being located on the celestial sphere. In his "Myth of Er", a section of the Republic, Plato describes the cosmos as the Spindle of Necessity, attended by the Sirens and turned by the three Fates. Eudoxus of Cnidus, who worked with Plato, developed a less mythical, more mathematical explanation of the planets' motion based on Plato's dictum stating that all phenomena in the heavens can be explained with uniform circular motion. Aristotle elaborated on Eudoxus' system.

In the fully developed Aristotelian system, the spherical Earth is at the center of the universe and all other heavenly bodies are attached to 56 concentric spheres which rotate around the Earth . (The number is so high because several transparent spheres are needed for each planet.) These spheres, known as crystalline spheres, all moved together at varying speeds to create the rotation of bodies around the Earth, and were composed of an intangible substance called aether. Aristotle believed that the moon was in the innermost sphere and therefore touches the realm of Earth, thus contaminating it, causing the dark spots (macula) and the ability to go through lunar phases. It is not perfect like the other heavenly bodies, which shine by their own light.[citation needed] He further described his system by explaining the natural tendencies of earth, water, fire, air, and aether. His system said that all earthly objects (solid objects) had a proclivity to move towards the Earth, water tended to remain on top of earth, air was the firmament between the water and fire, and that fire always tended to move in the opposite direction of earth (striving towards its natural position in the universe.) In addition, all celestial bodies in his system were composed of aether, the same material as the celestial spheres. This system therefore described why the earth was the center of the universe, and explained why the cosmos was arranged the way it seemed to be.

Adherence to the geocentric model stemmed largely from several important observations. First of all, if the Earth did move, then one ought to be able to observe the shifting of the fixed stars due to stellar parallax. In short, if the earth was moving the shapes of the constellations should change considerably over the course of a year. If they did not appear to move, the stars are either much further away than the Sun and the planets than previously conceived, making their motion undetectable, or in reality they are not moving at all. Because

the stars were actually much further away than Greek astronomers postulated (making movement extremely subtle), stellar parallax was not detected until the 19th century. Therefore, the Greeks chose the simpler of the two explanations. The lack of any observable parallax was considered a fatal flaw of any non-geocentric theory. Another observation used in favor of the geocentric model at the time was that apparent consistency of Venus' luminosity, thus implying that it is usually about the same distance from Earth, which is more consistent with geocentrism than heliocentrism. In reality, that is because the loss of light caused by its phases compensates for the increase in apparent size caused by its varying distance from Earth. Once again, Aristotle's objections of heliocentrism utilized his ideas concerning the natural tendency of earth like objects. The natural state of heavy earth-like objects is to tend towards the center of the earth and to not move unless forced by and outside object. It was also believed by some that if the Earth rotated on its axis, the air and objects in it (such as birds or clouds) would be left behind.

One major flaw in the Eudoxan and Aristotelian models based on concentric spheres was that they could not explain the changes in brightness of the planets caused by a change in distance.[4] Furthermore, the apparent inaccuracy of these systems became more prevalent over time, causing thinkers such as Ptolemy to posit new ideas and arrangements to better fit newly made observations.

Ptolemaic Model

Although the basic tenets of Greek geocentrism were established by the time of Aristotle, the details of his system did not become standard. The Ptolemaic system, espoused by the Hellenistic astronomer Claudius Ptolemaeus in the 2nd century AD finally accomplished this process. His main astronomical work, the Almagest, was the culmination of centuries of work by Hellenic, Hellenistic and Babylonian astronomers; it was accepted for over a millennium as the correct cosmological model by European and Islamic astronomers. Because of its influence, the Ptolemaic system is sometimes considered identical with the geocentric model.

Ptolemy argued that the Earth was in the center of the universe from the simple observation that half the stars were above the horizon and half were below the horizon at any time (stars on rotating stellar sphere) and the assumption that the stars were all at some modest distance from the center of the universe. If the Earth was substantially displaced from the center, this division into visible and invisible stars would not be equal.[5]

Ptolemaic system

The basic elements of Ptolemaic astronomy, showing a planet on an epicycle with an eccentric deferent and an equant point.

Pages from 1550 SACROBOSCO "Tractatus de Sphaera" with the Ptolemaic system.

In the Ptolemaic system, each planet is moved by a system of two or more spheres: one called its deferent, the others, its epicycles. The deferent is a circle whose center point exists halfway between the equant and the earth, marked by the X in the picture to the right where the equant is the solid point opposite the earth. Another sphere, the epicycle, is embedded inside of the deferent and is represented by the smaller dotted line to the right. A given planet then moves along the epicycle at the same time the epicycle moves along the path marked by the deferent. These combined movements cause the given planet to move closer to and further away from the Earth at different points in its orbit, and caused observers to believe that the planet even slowed down, stopped, and moved backward (in retrograde motion). This apparent retrograde motion was one of the largest inconsistencies in Greek cosmological systems, and was one of Ptolemy's main reasons for creating the deferent, epicycle model. The apparent retrograde motion was eventually replaced by the heliocentric model, and dispelled as an observation that is made only from earth bound observers. However, this model of deferents and epicycles made observations and predictions much more accurate than all preceding systems. The epicycles of Venus and Mercury are always centered on a line between Earth and the Sun (Mercury being closer to Earth), which explained why they were always near it in the sky.

The Ptolemaic order of spheres from Earth outward is:

1. Moon 2. Mercury 3. Venus 4. Sun 5. Mars 6. Jupiter 7. Saturn 8. Fixed Stars9. Sphere of Prime Mover

The deferent-and-epicycle model had been used by Greek astronomers for centuries, as had the idea of the eccentric (a deferent which is slightly off-center from the Earth). In the illustration, the center of the deferent is not the Earth but X, making it eccentric (from the Latin ex- or e- meaning "from," and centrum meaning "center"). Unfortunately, the system that was available in Ptolemy's time did not quite match observations, even though it was considerably improved over Aristotle's system. Sometimes the size of a planet's retrograde loop (most notably that of Mars) would be smaller, and sometimes larger. This prompted him to come up with the idea of an equant. The equant was a point near the center of a planet's orbit which, if you were to stand there and watch, the center of the planet's epicycle would always appear to move at the same speed. Therefore, the planet actually moved at different speeds when the epicycle was at different points on its deferent. By using an equant, Ptolemy claimed to keep motion which was uniform and circular, but many people did not like it because they did not think it was true to Plato's dictum of "uniform circular motion."[citation

needed] The resultant system which eventually came to be widely accepted in the west was an unwieldy one to modern eyes; each planet required an epicycle revolving on a deferent, offset by an equant which was different for each planet. But it predicted various celestial motions, including the beginnings and ends of retrograde motion, fairly well at the time it was developed.

Geocentrism and Islamic astronomy

Main articles: Maragheh observatory, Astronomy in medieval Islam, and Islamic cosmology

Due to the scientific dominance of the Ptolemaic system in Islamic astronomy, the Muslim astronomers accepted unanimously the geocentric model.[6]

In the 12th century, Arzachel departed from the ancient Greek idea of uniform circular motions by hypothesizing that the planet Mercury moves in an elliptic orbit,[7][8] while Alpetragius proposed a planetary model that abandoned the equant, epicycle and eccentric mechanisms,[9] though this resulted in a system that was mathematically less accurate.[10] Fakhr al-Din al-Razi (1149–1209), in dealing with his conception of physics and the physical world in his Matalib, rejects the Aristotelian and Avicennian notion of the Earth's centrality within the universe, but instead argues that there are "a thousand thousand worlds (alfa alfi 'awalim) beyond this world such that each one of those worlds be bigger and more massive than this world as well as having the like of what this world has." To support his theological argument, he cites the Qur'anic verse, "All praise belongs to God, Lord of the Worlds," emphasizing the term "Worlds."[11]

The "Maragha Revolution" refers to the Maragha school's revolution against Ptolemaic astronomy. The "Maragha school" was an astronomical tradition beginning in the Maragha observatory and continuing with astronomers from the Damascus mosque and Samarkand observatory. Like their Andalusian predecessors, the Maragha astronomers attempted to solve the equant problem (the circle around whose circumference a planet or the center of an epicycle was conceived to move uniformly) and produce alternative configurations to the Ptolemaic model without abandoning the geocentric model. They were more successful than their Andalusian predecessors in producing non-Ptolemaic configurations which eliminated the equant and eccentrics, were more accurate than the Ptolemaic model in numerically predicting planetary positions, and were in better agreement with empirical observations.[12] The most important of the Maragha astronomers included Mo'ayyeduddin Urdi (d. 1266), Nasīr al-Dīn al-Tūsī (1201–1274), Qutb al-Din al-Shirazi (1236–1311), Ibn al-Shatir (1304–1375), Ali Qushji (c. 1474), Al-Birjandi (d. 1525), and Shams al-Din al-Khafri (d. 1550).[13] Ibn al-Shatir, the Damascene astronomer (1304–1375 AD) working at the Umayyad Mosque, wrote a major book entitled Kitab Nihayat al-Sul fi Tashih al-Usul (A Final Inquiry Concerning the Rectification of Planetary Theory) on a theory which departs largely from the Ptolemaic system known at that time. In his book, "Ibn al-Shatir, an Arab astronomer of the fourteenth century," E.S.Kennedy wrote "what is of most interest, however, is that Ibn al-Shatir's lunar theory, except for trivial differences in parameters, is identical with that of Copernicus (1473–1543 AD)." The discovery that the models of Ibn al-Shatir are mathematically identical to those of Copernicus suggests the possible transmission of these models to Europe.[14] At the Maragha and Samarkand observatories, the Earth's rotation was discussed by al-Tusi and Ali Qushji (b. 1403); the arguments and evidence they used resemble those used by Copernicus to support the Earth's motion.[15][16]

However, the Maragha school never made the paradigm shift to heliocentrism.[17] The influence of the Maragha school on Copernicus remains speculative, since there is no documentary evidence to prove it. The possibility that Copernicus independently developed the Tusi couple remains open, since no researcher has yet demonstrated that he knew about Tusi's work or that of the Maragha school.[17][18]

Geocentrism and rival systems

This drawing from an Icelandic manuscript dated around 1750 illustrates the geocentric model.

Not all Greeks agreed with the geocentric model. The Pythagorean system has already been mentioned; some Pythagoreans believed the Earth to be one of several planets going around a central fire.[19] Hicetas and Ecphantus, two Pythagoreans of the 5th century BC, and Heraclides Ponticus in the 4th century BC, believed that the Earth rotated on its axis but remained at the center of the universe.[20] Such a system still qualifies as geocentric. It was revived in the Middle Ages by Jean Buridan. Heraclides Ponticus was once thought to have proposed that both Venus and Mercury went around the Sun rather than the Earth, but this is no longer accepted.[21] Martianus Capella definitely put Mercury and Venus in orbit around the Sun.[22] Aristarchus of Samos was the most radical. He wrote a work, which has not survived, on heliocentrism, saying that the Sun was at the center of the universe, while the Earth and other planets revolved around it.[23] His theory was not popular, and he had one named follower, Seleucus of Seleucia.[24]

Copernican system

Main article: Copernican heliocentrism

In 1543, the geocentric system met its first serious challenge with the publication of Copernicus' De revolutionibus orbium coelestium, which posited that the Earth and the other planets instead revolved around the Sun. The geocentric system was still held for many years afterwards, as at the time the Copernican system did not offer better predictions than the geocentric system, and it posed problems for both natural philosophy and scripture. The Copernican system was no more accurate than Ptolemy's system because it still used circular orbits. This was not altered until Johannes Kepler postulated that they may be elliptical, a belief that is still held to this day.

With the invention of the telescope in 1609, observations made by Galileo Galilei (such as that Jupiter has moons) called into question some of the tenets of geocentrism but did not seriously threaten it. Because he observed dark "spots" on the moon, craters, he was able to remark that the moon was not a perfect celestial body as had been previously conceived. This was the first time someone was able to see imperfections on a celestial body that was supposed to be composed from perfect aether. As such, because the moon's imperfections could now be related to those seen on Earth, one could argue that neither were unique, rather they were both just celestial bodies made from earth like material. Galileo was also able to see the moons of Jupiter, which he dedicated to Cosimo II de' Medici, and stated that they orbited around Jupiter, not Earth.[25] This was a significant claim because if it were true, it would mean that not everything revolved around Earth, shattering previously held theological and scientific thought. As such, Galileo's theories that challenged the geocentrism of our universe were silenced by the Church and general skepticism surrounding any system that did not place Earth at its center, preserving the thoughts and systems of Ptolemy and Aristotle.

Phases of Venus

In December 1610, Galileo Galilei used his telescope to observe that Venus showed all phases, just like the Moon. He thought that while this observation was incompatible with the Ptolemaic system, it was a natural consequence of the heliocentric system.

However, Ptolemy placed Venus' deferent and epicycle entirely inside the sphere of the Sun (between the Sun and Mercury), but this was arbitrary; he could just as easily have swapped Venus and Mercury and put them on the other side of the Sun, or made any other arrangement of Venus and Mercury, as long as they were always near a line running from the Earth through the Sun, such as placing the center of the Venus epicycle near the Sun. In this case, if the Sun is the source of all the light, under the Ptolemaic system:

If Venus is between Earth and the Sun, the phase of Venus must always be crescent or all dark.

If Venus is beyond the Sun, the phase of Venus must always be gibbous or full.

But Galileo saw Venus at first small and full, and later large and crescent.

This showed that with a Ptolemaic cosmology, the Venus epicycle can be neither completely inside nor completely outside of the orbit of the Sun. As a result, Ptolemaics abandoned the idea that the epicycle of Venus was completely inside the Sun, and later 17th century competition between astronomical cosmologies focused on variations of Tycho Brahe's Tychonic system (in which the Earth was still at the center of the universe, and around it revolved the Sun, but all other planets revolved around the Sun in one massive set of epicycles), or variations on the Copernican system.

Gravitation

Johannes Kepler, after analysing Tycho Brahe's famously accurate observations, constructed his three laws in 1609 and 1619, based on a heliocentric view where the planets move in elliptical paths. Using these laws, he was the first astronomer to successfully predict a transit of Venus (for the year 1631). The transition from circular orbits to elliptical planetary paths dramatically changed the accuracy of celestial observations and predictions. Because the heliocentric model by Copernicus was no more accurate than Ptolemy's system, new mathematical observations were needed to persuade those who still held on to the geocentric model. However, the observations made by Kepler, using the Brahe's data, became a problem not easily overturned for geocentrists.

In 1687, Isaac Newton devised his law of universal gravitation, which introduced gravitation as the force that both kept the Earth and planets moving through the heavens and also kept the air from flying away, allowing scientists to quickly construct a plausible heliocentric model for the solar system. In his Principia, Newton explained his system of how gravity, previously considered to be an occult force, conducted the movements of celestial bodies, and kept our solar system in its working order. His descriptions of centripetal force[26] were a breakthrough in scientific thought, and finally replaced the previous schools of scientific thought, i.e. those of Aristotle and Ptolemy. However, the process was gradual.

In 1838, astronomer Friedrich Wilhelm Bessel successfully measured the parallax of the star 61 Cygni, disproving Ptolemy's assertion that parallax motion did not exist. This finally substantiated the suppositions made by Copernicus with accurate, dependable scientific observations, and displayed truly how far away stars were from Earth.

A geocentric frame is useful for many everyday activities and most laboratory experiments, but is a less appropriate choice for solar-system mechanics and space travel. While a heliocentric frame is most useful in those cases, galactic and extra-galactic astronomy is easier if the sun is treated as neither stationary nor the center of the universe, but rotating around the center of our galaxy, and in turn our galaxy is also not at rest in the cosmic background.

Reluctance to change

While the evidence coming from the heliocentric camp was often rejected because its mathematical support was not strong enough, there were a large range of other reasons why people and institutions were not ready for such a monumental change in the way people viewed our universe. The Catholic Church for one found accepting the ideas of heliocentrism to be in conflict with many of their teachings and beliefs. If the Earth were God's creation, man, was not at the center of our universe, what did that mean? When referring to commonly used and believed cosmological arrangements, such as the strongly Christian influenced ones seen in Dante's Paradiso,[27] it was clear that the idea of Earth being the center of our universe was extremely ingrained in society at every level, not just scientific thinkers. The math was almost not even the most important aspect, just the sheer common sense of the system was compelling enough. And when beliefs that seemed so like common sense at the time were challenged by new theories, mainly heliocentrism, that were not even more accurate than the systems of Aristotle and Ptolemy, they were belittled and criticized at every level.

Some thinkers, such as St. Thomas Aquinas, had even melded the ideas of geocentrism into their theology. In his Summa Theologica, he says that the dual nature of Christ's rise to heaven was exhibited by his earthly body remaining on Earth, while his soul made of a different substance rose to heaven.[28] This was an attempt to directly agree with Aristotle's concept that objects composed from earthly substances tend towards the earth, while those composed of other materials, such as fire and aether, directed themselves away from the earth and rose up. Aquinas also discussed the arrangement of the firmament, or sky, compared to the arrangement of water and earth. As such, the Church's greatest thinkers had created a synthesis between theological teachings, and the scientific description of our universe's composition. This made a shift to a completely different system, centering the sun in our solar system, one that many people were not ready to make.

The importance of this resistance cannot be over-stressed. When figures such as Galileo were put on trial several times just to get him to recant, one is able to realize that the Church felt that it had much to lose. Also, Giordano Bruno's cosmological theories went beyond the Copernican model and identified the Sun as just one of an infinite number of independently moving heavenly bodies. He is the first European to have conceptualized the universe as a continuum where the stars we see at night are identical in nature to the Sun; for these ideas he was burned at the stake in 1600 after the Roman Inquisition found him guilty of heresy. After his death he gained considerable fame and in the 19th and early 20th centuries, commentators focusing on his astronomical beliefs regarded him as a martyr for free-thought and modern scientific ideas. Such efforts by the Church to suppress scientific thinking stalled its growth in general and slowed the switch from geocentrism to heliocentrism in particular.

Modern geocentrism

Main article: Modern geocentrism

The contemporary Association for Biblical Astronomy, led by physicist Dr. Gerardus Bouw, holds to a modified version of the model of Tycho Brahe, which they call geocentricity.[29]

Polls conducted by Gallup in the 1990s has found that 16% of Germans, 18% of Americans and 19% of Britons hold that the Sun revolves around the Earth.[30] A study done in 2005 by Dr. Jon D. Miller of Northwestern University, an expert in the public understanding of science and technology,[31] found that one adult American in five thinks the Sun revolves around the Earth.[32]

Planetariums

The geocentric (Ptolemaic) model of the solar system is still of interest to planetarium makers, as, for technical reasons, a Ptolemaic-type motion for the planet light apparatus has some advantages over a Copernican-type motion.[33] The celestial sphere, still used for teaching purposes and sometimes for navigation, is also based on a geocentric system.[34]

Geocentric models in science fiction

Alternate history science fiction has produced some literature of interest on the proposition that some alternate universes and Earths might indeed have laws of physics and cosmologies that are Ptolemaic and Aristotelian in design. This subcategory began with Philip Jose Farmer's short story, Sail On! Sail On! (1952), where Columbus has access to radio technology, and where his Spanish-financed exploratory and trade fleet sail off the edge of the (flat) world in his geocentric alternate universe in 1492, instead of discovering North America and South America.

Richard Garfinkle's Celestial Matters (1996) is set in a more elaborated geocentric cosmos, where Earth is divided by two contending factions, the Classical Greece-dominated Delian League and the Chinese Middle Kingdom, both of which are capable of flight within an alternate universe based on Ptolemaic astronomy, Aristotle's physics and Taoist thought. Unfortunately, both superpowers have been fighting a thousand-year war since the time of Alexander the Great.